High voltage stator coils use a conductive outer ground electrode to make certain the outer surface of the high voltage coils is at ground potential. These coils further include ground wall insulation generally comprised of multiple layers of glass backed mica paper tape. The outer ground electrode is defined by an outer layer of conductive tape loaded with carbon or graphite to control and set its conductivity. During a subsequent impregnation and heat cure process of the stator coil, shrinkage occurs in the conductive tape, which causes the tape to conform to the surface below it.
In some stator coils, the layer of conductive tape is wrapped in a half lap layer manner and applied over the last layer of mica paper tape that is also wrapped in a half lap layer. This method, while somewhat successful, causes protrusions of either the conductive tape or the mica layer.
The smoothness of the conductive tape layer has an influence on the electrical properties of the outer ground electrode. Protrusions of either the conductive tape or the mica tape below it can produce regions of locally enhanced electrical stress. Increased local electrical stress can cause locally produced partial discharges, which can lead to damage of the coil's outer ground electrode.